1. Field of the Invention
The present invention relates to a pinned layer structure having a nickel iron (NiFe) film for reducing the coercivity of a free layer structure in a spin valve sensor and, more particularly, to the nickel iron (NiFe) film acting as a seed layer for improving the sensitivity of a magnetic moment of the free layer structure to signal fields from a rotating magnetic disk.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
A read head in a magnetic disk drive of a computer includes the spin valve sensor as well as nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
Because of the interfacing of the pinning layer and the pinned layer structure the pinned layer structure is exchange coupled to the pinning layer. A unidirectional orientation of the magnetic spins of the pinning layer pins the magnetic moment of the pinned layer structure in the same direction. The orientation of the magnetic spins of the pinning layer are set by applying heat close to or above a blocking temperature of the material of the pinning layer in the presence of a field that is directed perpendicular to the ABS. The blocking temperature is the temperature at which all of the magnetic spins of the pinning layer are free to rotate in response to an applied field. During the setting, the magnetic moment of the pinned layer structure is oriented parallel to the applied field and the magnetic spins of the pinning layer follow the orientation of the pinned layer structure. When the heat is reduced below the blocking temperature the magnetic spins of the pinning layer pin the orientation of the magnetic moment of the pinned layer structure. The pinning function is effective as long as the temperature remains substantially below the blocking temperature.
As stated hereinabove, the magnetic moment of the free layer structure is free to rotate in response to signal fields from a rotating magnetic disk. The degree with which the magnetic moment is free to rotate in response to these signal fields equates to the sensitivity of the spin valve sensor. Accordingly, if the magnetic moment is stiff in its rotation by rotating only a small amount in response to a signal field the signal amplitude of the spin valve sensor is low because there has been a small amount of relative rotation between the pinned layer structure and the free layer structure. A measure of the stiffness of the free layer structure is by its easy axis coercivity HC or uniaxial anisotropy HK. The easy axis coercivity is the amount of field required to switch the orientation of the magnetic moment of the free layer structure 180xc2x0 along its easy axis while uniaxial anisotropy HK is the amount of field required to rotate the magnetic field 90xc2x0 from its easy axis. A free layer structure which consists entirely of a nickel iron (NiFe) free layer can have a low coercivity HC of only 1 or 2 Oe. It has become desirable, however, to combine a cobalt iron (CoFe) nanolayer (NL) with the nickel iron (NiFe) layer for the purpose of increasing the magnetoresistive coefficient dr/R of the spin valve sensor. Unfortunately, however, this addition raises the coercivity HC of the free layer structure up to about 10 Oe, thereby increasing the stiffness of the free layer structure in response to signal fields. There is a strong-felt need to maintain the improved magnetoresistive coefficient dr/R and reduce the coercivity HC of the free layer structure with the cobalt iron (CoFe) layer. In a dual spin valve sensor the free layer structure typically employs a nickel iron (NiFe) free layer between first and second cobalt iron (CoFe) layers. Accordingly, in the dual spin valve sensor the free layer structure is even more stiff in its operation because of the additional cobalt iron (CoFe) layer. Another problem that occurs when the coercivity HC of the free layer structure is high is that the rotation of the magnetic moment of the free layer structure in response to the signal field is not smooth. When this occurs the rotation is referred to as having jumps which causes noise in the playback system. The degree of coercivity HC of the pinned layer structure is also important which is discussed next.
In the presence of some magnetic fields the magnetic moment of the pinned layer structure can be rotated antiparallel to the pinned direction. The question then is whether the magnetic moment of the pinned layer structure will return to the pinned direction when the magnetic field is relaxed. This depends upon the strength of the exchange coupling field between the pinning layer and the pinned layer structure and the coercivity of the pinned layer structure. If the coercivity of the pinned layer structure exceeds the exchange coupling field between the pinning layer and the pinned layer structure the exchange coupling field will not be strong enough to bring the magnetic moment of the pinned layer structure back to the original pinned direction. Until the magnetic spins of the pinning layer are reset the read head is rendered inoperative. Accordingly, there is a strong felt need to decrease the coercivity of the pinned layer structure so that the exchange coupling field between the pinning layer and the pinned layer structure will return the magnetic moment of the pinned layer structure to its original orientation after being rotated therefrom.
As indicated hereinabove the strength of the exchange coupling field between the pinning layer and the pinned layer structure is also important in determining whether the pinned layer structure will return to its original orientation after being rotated therefrom. An antiferromagnetic material strongly considered for the pinning layer is nickel oxide (NiO) because it is nonmagnetic and will contribute to the read gap between the first and second shield layers by increasing the thickness of the first read gap layer. Further, since nickel oxide (NiO) is nonconductive it does not produce a sense current field on the free layer structure, upon the conduction of the sense current IS, which may be desirable in some embodiments for improving a balanced bias condition of the free layer structure. Unfortunately, however, nickel oxide (NiO) provides a low exchange coupling between the pinning layer and the pinned layer structure as compared to a pinning layer made from a metal, such as platinum manganese (PtMn), iridium manganese (IrMn) and nickel manganese (NiMn). Accordingly, it is desirable to use a pinning layer made of metal in lieu of nickel oxide (NiO).
My invention is employed in a spin valve sensor which has a metallic pinning layer. I have provided the pinned layer structure with a cobalt based film and a nickel iron (NiFe) film wherein the nickel iron (NiFe) film is located between the pinning layer and the cobalt based film. With this arrangement the nickel iron (NiFe) film of the pinned layer structure acts as a seed layer to lower the coercivity HC of the free layer structure. Accordingly, the increase in coercivity HC of the free layer structure due to the inclusion of one or more cobalt iron (CoFe) films or layers is mitigated by the nickel iron (NiFe) film in the pinned layer structure.
In another embodiment of the invention the pinned layer structure includes a nickel iron (NiFe) film which is located between first and second cobalt based films with the second cobalt based film interfacing the pinning layer. With this arrangement the microstructure of the nickel iron (NiFe) film is less susceptible to alteration by the microstructure of the pinning layer due to the intervention of the cobalt based film. The invention employs a metallic pinning layer which is preferably platinum manganese (PtMn). Accordingly, the invention provides the pinned layer structure with a nickel iron (NiFe) film in combination with a spin valve sensor that employs a metallic pinning layer. Previously a pinned layer structure of a spin valve sensor has been provided with a cobalt based film and a nickel iron (NiFe) film with the nickel iron (NiFe) film located between the cobalt based film and the pinning layer and interfacing the pinning layer for the purpose of isolating a nickel oxide (NiO) pinning layer from the cobalt based film. It was found that if the cobalt based film of the pinned layer structure interfaced the nickel oxide (NiO) pinning layer that a portion of the cobalt based film became an oxide of cobalt which increased its coercivity HC. Accordingly, the nickel iron (NiFe) film kept the coercivity HC of the cobalt based film from increasing due to oxidation. In contrast, the nickel iron (NiFe) film in the present invention decreases the coercivity HC of the cobalt based film in the pinned layer structure. With the nickel oxide (NiO) pinning layer there has been no data indicating that the nickel iron (NiFe) film, which interfaces the nickel oxide (NiO) pinning layer, also reduces the coercivity HC of the free layer structure. In contrast, it has been found that the present invention also reduces the coercivity HC of the free layer structure.
The invention is particularly useful in a dual spin valve sensor wherein the coercivity of the free layer structure is higher than that of a free layer structure in a single spin valve sensor. In the dual spin valve sensor a free layer structure is located between first and second spacer layers, the first and second spacer layers are located between first and second pinned layer structures and the first and second pinned layer structures are exchange coupled to first and second pinning layers. In the dual spin valve sensor the free layer structure has a higher coercivity HC due to the fact that a nickel iron (NiFe) layer is located between first and second cobalt iron (CoFe) layers. In the dual spin valve sensor embodiment each of the first and second pinned layer structures includes a cobalt based film and a nickel iron (NiFe) film. Preferably, each of the first and second pinned layer structures includes a nickel iron (NiFe) film which is located between first and second cobalt based films as described hereinabove.
In yet another preferred embodiment the pinned layer structure of the single spin valve sensor or the first and second pinned layer structures of the dual spin valve sensor are an antiparallel (AP) pinned layer structure wherein an AP coupling layer is located between first and second AP pinned layers. When the pinned layer structure is an AP pinned layer structure the first AP pinned layer, which is closest to the pinning layer, includes the aforementioned cobalt based film and the nickel iron (NiFe) film.
An object of the present invention is to provide a pinned layer structure of a spin valve sensor with a nickel iron (NiFe) film which reduces the coercivity of each of the pinned layer structure and a free layer structure in a spin valve sensor.
Another object is to provide a nickel iron (NiFe) film in a pinned layer structure which is exchange coupled to a metallic pinning layer for the purpose of serving as a seed layer for improving the performances of the pinned layer structure and the free layer structure.